Iris diaphragm devices of the kind cited above serve to regulate the beam cross section in optical instruments, for example in microscopes. The arrangement of the blades is implemented so that their one end is in each case rotatably supported on the annular mount, and their other end is in each case attached to the adjusting ring. As a result, the blades can be introduced as desired into the beam path by rotation of the adjusting ring, a central opening always remaining open. That central opening is steplessly adjustable between a maximum opening and a minimum opening.
These iris diaphragms known from the existing art thus cannot be completely closed, so that a residual portion of the beam cross section can always pass through. Other iris diaphragm devices having large iris diaphragms (e.g. of the Wilkes Iris company, see e.g. the Internet page www.wilkes-iris.com), in which a known iris diaphragm is combined with an additional specially shaped blade, are known for applications outside microscopy. Once the minimum opening of the iris diaphragm is reached, this specially shaped blade, constituting a spring-return diaphragm, is brought in front of the minimum opening by further actuation of the operating lever with the application of additional force.
Another iris diaphragm device (e.g. of the Wilkes-Iris company or the Edmund Optics company, see e.g. Internet page www.edmundoptics.com) has, in a very special design, two blade packets lying one behind another at a distance of a few millimeters. Because the minimum openings of the two blade packets are located at an offset behind one another, it is thereby possible to achieve complete closure of this special iris diaphragm device. Although in this fashion light of the beam cross section no longer passes through the iris diaphragm device, a defined diaphragm plane does not exist in this case because of the two blade packets in different planes. This proves, however, to be extremely unsuitable for microscopy, in which a single defined diaphragm plane is necessary.